<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    * {
      margin: 0;
      padding: 0;
    }

    .father {
      width: 400px;
      height: 400px;
      background-color: pink;

      /* border-top: 1px solid #000; */
      /* padding-top: 1px; */

      /* overflow: hidden; */

      /* display: inline-block; */

      float: left;
    }

    .son {
      width: 100px;
      height: 100px;
      background-color: blue;
      margin-top: 100px;
    }

  </style>
</head>
<body>
  <div class="father">
    <!-- 我是一行文本 -->
    <div class="son"></div>
  </div>
</body>
</html>